This invention relates to a camera and prism assembly used for viewing images of specimens in an electron microscope, and more particularly to a camera which is mounted externally of the microscope and which can be readily adjusted about a horizontal axis for aligining such images on a viewing device.
Transmission electron microscopes use a beam of accelerated electrons which pass through a specimen to provide an electron image and/or diffraction pattern of the specimen. To provide a record of these images and/or diffraction patterns, the electrons have been converted into light images using scintillator materials (e.g., phosphors), and the light images and/or patterns are then captured by a camera. While photographic film has long been used, charge-coupled devices (CCD) of the type originally developed for astronomy to read light images into a computer have found increasing use in this field. Such CCD cameras offer excellent resolution, sensitivity, linearity, are reusable, and make the image available for viewing within seconds of recording.
With the advent of digital image processing in transmission electron microscopy, such CCD cameras have been used to transfer images directly from the microscope to a computer. Typically, the electron image is converted to a light image by means of a scintillator such as a phosphor screen mounted inside the transmission microscope. The light image is then transferred to a CCD camera, also located inside of the microscope, using a lens or fiber optic coupling. The image collected by the camera is then digitized electronically and stored in a computer where it be manipulated and viewed with the aid of appropriate software.
In order to acquire an electron image, a scintillator such as a phosphor screen, must first be inserted into the path of the image. For viewing a wide image area, the phosphor screen is positioned above the normal fluorescent viewing screen and close to the microscope projector lens. The phosphor screen must also be retractable so that it does not shadow other devices, such as an electron spectrometer or a film camera, that are typically mounted further down the transmission electron microscope column.
One problem which has affected prior art systems is that the mechanism which inserts and retracts the phosphor screen must operate smoothly so that it does not transmit vibrations to the microscope which could reduce image resolution. Further, after each cycling of the screen between an inserted and a retracted position, the mechanism must be able to return the phosphor screen precisely (to within one image pixel of resolution at the CCD camera) to its previous location. Otherwise, a new reference image must be acquired with each cycle so that computer-controlled gain normalization can be carried out on the image.
Prior art devices which use mechanical screw drive mechanisms have reproducibility problems because of the back lash and dead travel inherent in these mechanisms. Further, such prior mechanisms could not be interfaced with any automatic control apparatus as there was no ability to predict or measure the amount of back lash compensation needed each time the phosphor screen was moved.
Current CCD cameras are typically positioned in a fixed location, for example, mounted on the base of the electron microscope projection chamber. Such a fixed location sacrifices flexibility, as trade-offs must be made between enhancing resolution of the acquired image and maximizing light collection efficiency for faint images. Further, the fixed location of prior art CCD cameras restricts how the captured image may be viewed by the user.
Further, mounting the camera in a vacuum within the electron microscope presents problems of maintaining low operating temperatures (e.g., about -30.degree. C.) and the degree of vacuum over time. Krivanek, U.S. Pat. No. 5,065,029, addresses that problem by providing a vacuum valve which opens and closes to separate the camera from the electron microscope chamber. In another embodiment, the patentee teaches a movable camera mounted on a pneumatically-operated piston which can be inserted into the electron microscope chamber and then withdrawn and sealed off. However, the camera still returns to the same location after each cycle and cannot be adjusted to a different location.
Accordingly, the need exists in this art for a camera and viewing system for an electron microscope which provides for better automatic control, improved precision, and greater flexibility in acquiring and displaying specimen images.